The present invention relates to a method and apparatus for predicting the unbalance condition of a load of material in an appliance and more particularly, for predicting an unbalance condition of a load of material in a rotatable vessel of the appliance.
Various appliances, such as automatic washing machines, automatic dryers, centrifugal liquid extractors, etc., utilize a rotating tub, basket or other vessel holding a load of material which may or may not be evenly distributed within the vessel. The condition of having the load unevenly distributed, or out of balance, creates a situation where the center of mass of the rotating vessel does not correspond to the geometric axis of the vessel. This leads to the generation of high loads and severe vibration of the vessel. In an appliance, this severe vibration may cause the phenomenon of movement of the appliance across the floor or other supporting surface. This can occur both in vertical axis rotating vessels as well as horizontal axis vessels and also in those appliances where the axis is arranged in between vertical and horizontal.
Various attempts have been provided in the prior art to provide mechanical arrangements to limit or reduce the possibility of unbalanced loads, which typically involve the addition of various masses, either fixed or movable, to the vessel which requires additional power for the motor to rotate the vessel.
Approaches have also been disclosed in the prior art for detecting a load imbalance, for example, in an inverter driven motor for a washing machine, as disclosed in U.S. Pat. No. 5,070,565. That patent discloses to examine a ripple in the dc-inverter bus current, with a ripple value above a predetermined level being indicative of load unbalance. If a load unbalance is detected, the washer controller would resume a redistribution cycle to attempt to re-balance the clothes. This would be attempted a predetermined number of times and, if the load is still unbalanced, the spin cycle would be aborted. If the ripple value falls below the pre-determined level before the maximum number of tries is reached, the spin cycle is started. Once a spin cycle has been initiated, the length of the spin cycle is determined on the basis of the magnitude of any remaining load unbalance. Spin rate and spin time may be adjusted based upon the degree of load unbalance detected.
It would be an advance if a method and apparatus were provided in which the potential for a severe unbalance could be predicted in advance of it actually occurring so that appropriate steps could be taken to avoid the detrimental effects of such a condition.
The present invention provides a method and apparatus for predicting, at a relatively low rotational speed, a severe unbalance condition in a rotating device such as a basket, tub or other rotatable vessel of an appliance, for example an automatic washer. The method and apparatus provide the prediction by monitoring the motor current signature. When the amount of unbalance is estimated at a low rotational speed, the cycle can attempt a redistribution routine in order to eliminate the unbalance condition before it becomes a problem. If unbalance still persists, the spin speed can be adaptably limited or the cycle can be terminated and the user can be advised.
The effect of unbalanced loads in a motor driven rotating component, such as a rotatable vessel, translates into motor torque oscillations, which are proportional to the motor stator currents. Moreover, increased vibrations in certain appliances cause energy dissipation in passive components, such as in the suspension system, causing the average motor current to increase. In the case of a controlled induction motor (CIM), the stator currents are estimated by directly measuring the dc bus current of the inverter.
In the present invention, motor torque oscillations are monitored at low speed and a severe unbalance condition is predicted before it develops into a problem condition.
A special speed profile is commanded to the motor by the control system in order to obtain information about the load. When a steep acceleration is applied at low speed, such as an increase from 60 rpm to 100 rpm in approximately 1 second, the presence of large unbalances in the vessel makes the vessel hit the cabinet, causing perturbations (xe2x80x9cbumpsxe2x80x9d) in the motor torque and current. It has been observed experimentally that these perturbations are proportional to the amount of unbalanced load present in the vessel and relate to the extremely unbalanced vibrational behavior of the appliance at higher rotational speeds.
The apparatus may be arranged and selected such that the vessel itself is not striking the cabinet, however, some component which moves with the vessel should preferably engage with some component which is relatively stationary as compared to the cabinet. In this manner movement of the vessel relative to the cabinet (other than rotational) can be detected and measured. Thus, as used herein, and including in the claims, the concept of the vessel striking the cabinet is intended to include such vessel components engaging such cabinet components.
A faster motor frequency and a slower bump frequency characterize the current signature. More accurately, the motor current has three components, two of which are harmonic. A first component is the nominal motor current. The second component is the frequency that is input into the motor to determine its fundamental speed. The third component is created by the motor when it responds with increases in motor torque that are required to overcome the gyroscopic effects of the vessel striking the cabinet as the motor tries to maintain constant speed. Nominal motor current and motor frequency go into the motor which sets the motor running at a constant speed. When the vessel hits the cabinet it tries to slow the vessel down, and the motor increases torque to prevent this from happening. What results is the sum of nominal motor current, the motor frequency and the frequency with which the vessel strikes the cabinet. In order to extract the unbalance information, the motor frequency is digitally filtered out with a running average algorithm. This leaves the bump frequency component and the nominal motor current. The bump frequency is then filtered out, leaving a nominal motor current curve. The difference between the nominal motor current curve and the curve with the bump frequency is integrated to obtain a measure of the energy used by the motor to maintain constant speed when the vessel strikes the cabinet. This is termed bump energy. The bump energy is accumulated for a fixed amount of time, for example a few seconds, and is then compared to a threshold in order to determine whether a higher rate spin cycle should proceed or whether some corrective action should be taken.